Brush cutters, in general, have a handle for its user to grip. When a worker works for cutting grass or shrubs with the brush cutter, he/she moves both hands gripping the handle right and left to move the cutting blade, in high speed rotation, right and left. Known brush cutters include those of engine-driven ones driven by an internal combustion engine and motor-driven ones driven by an electric motor. From another standpoint, known brush cutters include those of a backpack type and those of a shoulder-strap type. In operation with a backpack type brush cutter, the user will carry its main body containing its drive source on his/her back. In operation with a shoulder-strap type brush cutter, the user will carry the main body on his/her shoulder with a strap extending from the main body of the brush cutter.
Brush cutters are work apparatuses having a cutting blade. For this reason, there have been developed brush cutters equipped with a safety device intended to protect the worker using the brush cutter from its cutting blade in order to assure his or her safety when any dangerous condition arises suddenly during the work.
Japanese Utility Model Laid-open Publication No. H02(1990)-131822 (hereafter referred to as “Patent Document 1” as well) discloses a shoulder-strap type brush cutter. On each of right and left handles of the brush cutter, a stop lever is provided. The stop lever is actuated when grasped by the worker. When the worker takes his/her hand off at least one of the right and left stop levers, namely, when the stop lever is released, a fuel valve mechanically linked to the stop lever is closed, and forcibly stops fuel supply to the engine or supply of power to an ignition plug of the engine to thereby stop the engine from running.
Japanese Patent Laid-open Publication No. H04-(1992)-158714 (hereafter referred to as “Patent Document 2” as well) discloses a shoulder-strap type brush cutter in which a stop lever or pressure sensor is provided on a handle of the brush cutter to actuate an electromagnetic brake or mechanical stopper and thereby forcibly stop the rotation of the cutting blade when the worker unlinks hands from the handle.
Japanese Patent Laid-open Publication No. 2006-288296 (hereafter referred to as “Patent Document 3” as well) discloses a hand-held type electric brush cutter having a shock sensor. The shock sensor includes a piezoelectric element that abuts a circumferential part of a manipulation tube provided around a power transmission shaft connecting an electric motor or an engine as a drive source to a cutting blade. When the shock sensor detects a shock larger than a predetermined level, the motor is forcibly stopped. More specifically, a box surrounding the manipulation tube is provided near a handle fixed at a lengthwise central portion of the manipulation tube, and the shock sensor is affixed to the box in abutment with the manipulation tube. In this Patent Document 3, it is also proposed to provide, in the safety device including the shock sensor, a variable resistor for adjusting a current flowing through the shock sensor (piezoelectric element) to eliminate the variation in sensitivity from one piezoelectric element to another.
Japanese Patent Laid-open Publication No. H08(1996)-187024 (hereafter referred to as “Patent Document 4” as well) discloses a shoulder-strap type brush cutter including an internal combustion engine or electric motor as a drive source and a centrifugal clutch interposed between the drive source and a power transmission shaft mechanically connecting the drive source and a cutting blade. The centrifugal clutch includes a clutch shoe that is mechanically activated by a large relative displacement, if any, between the drive source and the power transmission shaft to automatically brake the centrifugal clutch.
Japanese Patent Laid-open Publication No. 2008-118960 (hereafter referred to as “Patent Document 5” as well) discloses a shoulder-strap type brush cutter. The strap for a worker to carry the brush cutter on his/her shoulder has a cap at its distal end, and it can detachably engage with a switch main body of the brush cutter. In this shoulder-strap type brush cutter, if the cutting blade hits a large stone or the like and the brush cutter itself is kicked back, the cap is disengaged from the switch body, and power supply to the ignition plug is forcibly interrupted to stop the engine.
Japanese Patent Laid-open Publication No. S62(1988)-198320 (hereafter referred to as “Patent Document 6” as well) proposes to provide a mercury switch that detects an angle of inclination of a manipulation tube provided around a power transmission shaft through which the output of an electric motor is transmitted to a cutting blade. In this proposal, once the detected inclination angle of the manipulation tube gets out of a predetermined range, power supply to the electric motor is interrupted to forcibly stop the electric motor.